a) Field of the Invention
The invention is directed to a system for contactless measurement of the optical imaging quality of an eye with an interferometer by which at least one light pulse with a short coherence length is coupled into the eye from a light source.
b) Background of Problems Relating to the Invention
Following operations in the region of the lens of the eye and the comea, for example, cataract operations or refractive procedures on the comea, optical deviations or aberrations can occur in the visual system of the eye which can not be corrected by spherical or astigmatic lenses or cylindrical lenses. Another problem with these aberrations is that they increase dramatically as the pupil diameter increases and thus seriously impair night vision in particular.
Therefore, it is important to detect these aberrations by measurement techniques in order to be able to initiate appropriate corrective measures already during the operation if possible. Further, in the case of cataract operations in which the original lens has been removed and replaced by an artificial lens, it is very important to measure the length of the eye in order to select an appropriate synthetic replacement lens which is adapted to the length of the eye and thus enables good vision.
Since these measurements of the imaging quality of the eye have an effect on the lens in its interaction with the eye length as well as on the aberrations and are to be carried out during and/or shortly after the operation, only noncontacting or contactless methods may be applied for the sake of quality assurance in order to prevent tissue irritation and infection. Therefore, the measurement of the length of the eye by ultrasound as was commonly performed heretofore is ruled out because it requires an ultrasound head to be placed on the cornea.
c) Description of the Related Art
The article “Optical Coherence Tomography” by A. Fercher, Journal of Biomedical Optics, Vol. 1, No. 2, April 1996, 157–173, describes a method for analyzing the length of the eye in contactless mode.
By means of a two-arm interferometer, e.g., a Michelson interferometer, light is coupled into the eye to be measured as continuous light or at least in the form of short light pulses or wavetrains. Interference can be brought about between a light reflection originating from the cornea and a light reflection originating from the retina in the area of a sensing device by means of a given difference in arm length of the interferometer which approximately corresponds to the anticipated length of the human eye, usually between 24 mm and 28 mm. A reflector in the area of the interferometer can be measured by a measuring device, or scanner, until the desired interference pattern between the retinal and corneal reflections comes about. The measurable movement path required for this purpose together with the known starting position and the known preserved difference in arm length of the interferometer gives a quantity which makes it possible for accurate conclusions to be made about the length of the measured eye, that is, the distance between the surface of the cornea and the surface of the retina.
However, it is disadvantageous that only the length of the eye can be measured by this approach.
U.S. Pat. No. 5,975,699 describes a method and a device which simultaneously measures the length of the eye and the refractive error. This device, which is capable of measuring the length of the eye and the refractive error simultaneously, couples light into the interior of the eye via a Michelson interferometer with a predetermined length difference between the two intersecting optical arms and via an additional beam splitter. The corneal and retinal reflections then arrive partly unused in the interferometer via this beam splitter; the other part which is used for measurement arrives, via an optical imaging device, at a grating where a spectral division of the light takes place. Subsequently, a spectral evaluation of this light is carried out in order to determine the refractive error of the eye and to evaluate the intensity variations of the corneal and retinal reflections generated by the interference due to the different path lengths of the interferometer in order to determine the length of the eye.
Both of the methods mentioned above have in common that they are incapable of detecting greater deviations of the visual system through measurement techniques. In addition, an external interferometer with an arm length difference corresponding to the optical length of the eye to be investigated is applied in both methods. A basic feature of the interferometer consists in that fifty percent of the light is lost at the beam splitter and good perceptibility of the signals is impaired by the resulting attenuation of the intensity of the light and of the reflections introduced. Moreover, a Michelson interferometer is complicated to set up and handle.
A method for analyzing optical deviations in an eye is described in the article “Measurement Equipment for Determining the Monochromatic Aberration of the Human Eye” by P. Mierdel, H. E. Krinke, W. Wiegand, M. Kaemmerer, T. Seiler, Der Ophthalmologie, Vol. 94, June 1997, 441–445.
In this case, a uniform pattern of light points is generated on the fundus oculi in the area of the retina of the eye to be examined by focusing light beams generated by a perforated mask through a collector lens at a short distance from the retina. In an ideal eye, this causes a uniform pattern of light points on the retina. In an eye with corresponding aberrations in the region of the lens or cornea, however, a distorted pattern of light points is brought about on the retina. An intermediate image of the retinal light point pattern is then generated via corresponding optical imaging devices and is imaged on a light-sensitive CCD sensor by a camera objective. The imaging of the light point pattern is distorted when the visual system has optical aberrations. These aberrations can be numerically analyzed. The results are displayed as a list of weighting factors of Zernike polynomials by which a wavefront aberration topography can be modeled.
Another method is described in the article “Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave front sensor” by J. Liang, B. Grimm, S. Goelz, J. F. Bille, J. Opt. Soc. Am. A, Vol. 11, No. 7, July 1994.
By means of a reflection of introduced light, a secondary light source in the area of the retina is generated on the fundus of the eye to be examined. The light beam bundle of the retinal reflection is then concentrated on a CCD target by a lens arrangement, or lens array, as it is called. The light beam bundle exiting from the pupil is composed of parallel rays in an emmetropic eye, that is, an ideal or healthy eye without aberrations. Therefore, in an emmetropic eye the rays which are bundled through the lens array show a uniform grid-like pattern of light points. When there are aberrations in the visual system, individual rays from the bundle of rays have deviations from their ideal direction or parallelism because of the deviation of the wavefront of the light exiting from the pupils of the eyes. Thus, in an eye with aberrations, the light point pattern deviates from the uniform pattern of the emmetropic eye. These deviations can then be numerically analyzed in order to obtain weighting factors of Zernike polynomials.
The last two methods mentioned above make it possible to measure aberrations of the visual system. However, they are disadvantageously limited to measurement of these aberrations only and can not measure the length of the human eye.